Is information lost in wavefunction collapse?

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Discussion Overview

The discussion revolves around the concept of information loss in the context of wavefunction collapse in quantum mechanics, particularly in relation to the black hole information paradox. Participants explore the implications of wavefunction collapse, decoherence, and the nature of measurement within quantum mechanics, considering various interpretations and their consequences.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants suggest that wavefunction collapse may represent a loss of information, particularly when only one classical outcome is observed, contrasting it with the reversible nature of decoherence.
  • Others argue that wavefunction collapse is an interpretation of quantum mechanics that does not yield different predictions from standard quantum mechanics, which allows for no-collapse interpretations.
  • A participant questions whether the collapse is reversible, linking the concept of information loss to time non-reversibility in physics.
  • Some participants assert that standard quantum mechanics does not provide a precise definition of "measurement," leading to multiple interpretations regarding what occurs during measurement.
  • There is a discussion on the implications of a specific measurement scenario, such as a circularly polarized photon passing through a polarizer, and whether this process can be considered reversible.
  • One participant expresses skepticism about the superposition interpretation, arguing that observing a specific outcome in a closed system raises questions about the fate of the information.
  • Another participant introduces the idea that if information can be lost in wavefunction collapse, it could relate to the arrow of time and entropy, suggesting a potential connection that is not commonly discussed.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether information is lost during wavefunction collapse. Multiple competing views are presented regarding the nature of collapse, measurement, and the implications for information theory in quantum mechanics.

Contextual Notes

Participants highlight the ambiguity surrounding the definitions of measurement and collapse in standard quantum mechanics, as well as the unresolved nature of whether wavefunction collapse is a real physical process or merely an interpretative framework.

  • #61
PeterDonis said:
No, it doesn't. The last sentence of that post highlights the issue: standard QM does not specify where the information has gone. But that doesn't mean the information is lost, or that it's not lost. It just means standard QM can't tell you whether it's lost or not.
Is this the same as saying the there is no observable (self-adjoint operator) in standard QM that can be attributed to that which has/has not been lost ?
 
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  • #62
PeterDonis said:
If that is the case, then I don't think it's correct to describe the standard QM collapse as non-unitary.

PeterDonis said:
No, it doesn't. The last sentence of that post highlights the issue: standard QM does not specify where the information has gone. But that doesn't mean the information is lost, or that it's not lost. It just means standard QM can't tell you whether it's lost or not.

I understand where you are coming from, and the more general sense of "information" in plain English. However, "information loss" in the black hole information paradox is one of those physics jargon terms that can be misleading for the general public, like "work" in Newtonian Mechanics or "observer" in special relativity.

Th black hole information paradox is that reasonable postulates lead to a loss of unitarity incompatible with standard QM. The most common approaches (AdS/CFT) to solving the paradox have to do with quantum gravity, and nothing to do with the measurement problem, and aim to restore unitarity in the framework of standard QM.
 
  • #63
atyy said:
Standard QM has collapse - see the texts by Dirac, Landau and Lifshitz, Cohen-Tannoudji et al, Weinberg, Sakurai, Griffiths.

For many years THE standard text on QM was Dirac which I have a copy of. It has a few issues but not related to this. What standard QM is can be found on page 45 under the heading of - The General Physical Interpretation. His assumption is given an observable O and a state x the average of making the observation associated with O, E(O) is E(O) = <x|O|x> .

Now I did not go through the whole book to see if he uses the word collapse anywhere, but it is not in his general physical Interpretation. And the above is all you need to solve problems.

It is often thought Dirac was in the Copenhagen School of Neil's Bohr - but in actual fact he wasn't - although its hard to find evidence of it because for him math was the thing - interpretations were not much of an issue - and he was notoriously a man of few words. That said, from what he did write, he had a very subtle view of QM and physics in general - here he is arguing with Heisenberg about one of the tenants of Copenhagen - that the state is a complete description of the system and it has reached it's final form:
http://philsci-archive.pitt.edu/1614/1/Open_or_Closed-preprint.pdf
'Dirac criticized the Copenhagen theorists for claiming that quantum theory had attained its final form. In a 1929 letter to Bohr he writes 'I am afraid I do not completely agree with your views. Although I believe that quantum mechanics has its limitations and will ultimately be replaced by something better, . . . I cannot see any reason for thinking that quantum mechanics has already reached the limit of its development. I think it will undergo a number of small changes.'

Thanks
Bill
 
  • #64
bhobba said:
For many years THE standard text on QM was Dirac which I have a copy of. It has a few issues but not related to this. What standard QM is can be found on page 45 under the heading of - The General Physical Interpretation. His assumption is given an observable O and a state x the average of making the observation associated with O, E(O) is E(O) = <x|O|x> .

Now I did not go through the whole book to see if he uses the word collapse anywhere, but it is not in his general physical Interpretation. And the above is all you need to solve problems.

It is often thought Dirac was in the Copenhagen School of Neil's Bohr - but in actual fact he wasn't - although its hard to find evidence of it because for him math was the thing - interpretations were not much of an issue - and he was notoriously a man of few words. That said, from what he did write, he had a very subtle view of QM and physics in general - here he is arguing with Heisenberg about one of the tenants of Copenhagen - that the state is a complete description of the system and it has reached it's final form:
http://philsci-archive.pitt.edu/1614/1/Open_or_Closed-preprint.pdf
'Dirac criticized the Copenhagen theorists for claiming that quantum theory had attained its final form. In a 1929 letter to Bohr he writes 'I am afraid I do not completely agree with your views. Although I believe that quantum mechanics has its limitations and will ultimately be replaced by something better, . . . I cannot see any reason for thinking that quantum mechanics has already reached the limit of its development. I think it will undergo a number of small changes.'

Thanks
Bill

Dirac has collapse.
 
  • #65
atyy said:
Dirac has collapse.

I could be wrong - but I could not find it in his text - can you give the page number?

Thanks
Bill
 
  • #66
bhobba said:
I could be wrong - but I could not find it in his text - can you give the page number?

Thanks
Bill

In the 4th edition, it is on p36.
 
  • #67
atyy said:
In the 4th edition, it is on p36.

No - he says - the measurement causes the system to jump to an eigenstate after the measurement. And he also uses the physical continuity argument I have mentioned many times to derive it must jump ie be in that sate immediately AFTER the measurement. Nobody argues it is in the eigenstate immediately after the measurement - its the specific collapse postulate we are talking about. Collapse has a stronger meaning than this - it means unitary evolution is broken and it discontinuously changes the state - see page 330-331 of Schlosshauer's textbook I am always mentioning - Decoherence and the Quantum to Classical Transition. The fact is we do not know if it is discontinuous or not - we only know it is different AFTER the measurement. Whats going on during the measurement is unknown - it is an interpretation to say it's discontinuous.

In fact decoherence suggests it is not discontinuous - but we really do not know. MW would indeed say it is not discontinuous. In collapse theories like GRW is does indeed happen spontaneously and presumably discontinuously.

Thanks
Bill
 
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  • #68
bhobba said:
No - he says - the measurement causes the system to jump to an eigenstate after the measurement.

Those words are ambiguous---they could be given a "disturbance" interpretation, which doesn't seem like collapse:
  • If you try to measure the energy of a bound electron, the interaction between measuring device and electron will result in the electron being forced into an energy eigenstate.
However, if you have an entangled pair of particles (as with EPR), then measuring a property of one particle can seemingly cause the other particle to collapse into an eigenstate of whatever is being measured. The collapse of the distant particle can't be given a disturbance interpretation (without FTL influences).

So I don't think that Dirac's nuanced distinction between "collapse" and "measurement causing the system to jump to an eigenstate" really helps. If the latter is true, it sure seems to me that the former is, also.
 
  • #69
bhobba said:
No - he says - the measurement causes the system to jump to an eigenstate after the measurement. And he also uses the physical continuity argument I have mentioned many times to derive it must jump ie be in that sate immediately AFTER the measurement. Nobody argues it is in the eigenstate immediately after the measurement - its the specific collapse postulate we are talking about. Collapse has a stronger meaning than this - it means unitary evolution is broken and it discontinuously changes the state - see page 330-331 of Schlosshauer's textbook I am always mentioning - Decoherence and the Quantum to Classical Transition. The fact is we do not know if it is discontinuous or not - we only know it is different AFTER the measurement. Whats going on during the measurement is unknown - it is an interpretation to say it's discontinuous.

In fact decoherence suggests it is not discontinuous - but we really do not know. MW would indeed say it is not discontinuous. In collapse theories like GRW is does indeed happen spontaneously and presumably discontinuously.

Thanks
Bill

I disagree. Dirac does mean collapse.

As if there were any ambiguity, p108 further shows that this is what he meant.
 
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  • #70
stevendaryl said:
Those words are ambiguous---they could be given a "disturbance" interpretation, which doesn't seem like collapse

You are falling for the same trap. What Dirac calls a jump is a simple deduction of the Born Rule. Collapse says more. In EPR we know its a correlation and like any 100% correlation as soon as you know one you know the other. In the classical envelope analogy does the other envelope suddenly collapse - of course not. The only difference in QM is it has different statistical properties - but something may or may not have discontinuously changed - we simply do not know. To be specific entanglement is broken - does that happen instantaneously - its the same as any observation - we do not know.

Thanks
Bill
 
  • #71
bhobba said:
You are falling for the same trap. What Dirac calls a jump is a simple deduction of the Born Rule. Collapse says more. In EPR we know its a correlation and like any 100% correlation as soon as you know one you know the other. In the classical envelope analogy does the other envelope suddenly collapse - of course not. The only difference in QM is it has different statistical properties - but something may or may not have discontinuously changed - we simply do not know. To be specific entanglement is broken - does that happen instantaneously - its the same as any observation - we do not know.

Thanks
Bill

In the classical case, there is a sudden "collapse" representing a change in your knowledge. So it is not true that there is no discontinuity in the classical case.
 
  • #72
atyy said:
In the classical case, there is a sudden "collapse" representing a change in your knowledge. So it is not true that there is no discontinuity in the classical case.

The issue is not that your knowledge changes - if course it does. The issue is it discontinuous. Imagine opening the envelope - you don't open it and notice its color instantaneously and discontinuously - it takes time to register for example. This is the precise issue - collapse says it happens non unitaryily and discontinuously - we don't know it does that - it may or may not.

Thanks
Bill
 
  • #73
bhobba said:
You are falling for the same trap. What Dirac calls a jump is a simple deduction of the Born Rule. Collapse says more.

I don't see that it does say more.

[edit: added]

If you say that AFTER a measurement, a system is in such-and-such a state, then it seems to me that are two possibilities:
  1. It was in that state before the measurement, and the measurement just informed you of this fact.
  2. The measurement process put it into that state.
Number 1. is impossible by Bell's theorem. Number 2 is collapse.

MWI actually rejects the premise: The fact that I measure the system to be in a state doesn't imply that it is in that state (or at least not exclusively---in some other "world", it's in a different state).

In EPR we know its a correlation and like any 100% correlation as soon as you know one you know the other. In the classical envelope analogy does the other envelope suddenly collapse - of course not.

Yes, and Bell's proof shows that EPR correlations are nothing like that.
 
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  • #74
stevendaryl said:
Yes, and Bell's proof shows that EPR correlations are nothing like that.

It says if you want it like classical correlations you need non locality - it says nothing about if entanglement is broken instantaneously or not.

Thanks
Bill
 
  • #75
bhobba said:
It says if you want it like classical correlations you need non locality - it says nothing about if entanglement is broken instantaneously or not.

I don't know what it means for entanglement to be broken instantaneously or not instantaneously.
 
  • #76
stevendaryl said:
I don't know what it means for entanglement to be broken instantaneously or not instantaneously.

When you observe one part of an entangled pair at the end of the observation we know what we observed and the entanglement with what it is entangled with is broken. But what is going on during the observation to that entanglement - does it change instantaneously and discontinuously or is something else going on? We do not know.

Thanks
Bill
 
  • #77
bhobba said:
When you observe one part of an entangled pair at the end of the observation we know what we observed and the entanglement with what it is entangled with is broken. But what is going on during the observation to that entanglement - does it change instantaneously and discontinuously or is something else going on? We do not know.

I don't quite understand what it is that you're saying might be changing continuously. Let's make it concrete: We have a source of anti-correlated spin-1/2 pairs. We have two distant experimenters, Alice and Bob. Alice measures spin-up along the z-axis at time ##t##. Then she knows instantly the following fact about Bob: "If Bob measures the spin of his particle along the z-axis, he will measure spin-down." I don't see how continuous versus noncontinuous evolution is relevant. There definitely isn't a time window in which Bob might get a different answer, so the breaking of the entanglement doesn't propagate slowly in that sense.
 
  • #78
atyy said:
As if there were any ambiguity, p108 further shows that this is what he meant.

There is no ambiguity. He says the state changes unpredictably. Nobody disagrees with that. Its the other baggage associated with collapse that is the issue.

MW proves it does not have to happen using non-unitary changes and instantaneously, nor does the formalism require it to be. It may be like that or not - we do not know. It may be like GRW - again we do not know. The formalism is silent on it.

Thanks
Bill
 
  • #79
stevendaryl said:
I don't quite understand what it is that you're saying might be changing continuously. Let's make it concrete: We have a source of anti-correlated spin-1/2 pairs. We have two distant experimenters, Alice and Bob. Alice measures spin-up along the z-axis at time ##t##. Then she knows instantly the following fact about Bob: "If Bob measures the spin of his particle along the z-axis, he will measure spin-down." I don't see how continuous versus noncontinuous evolution is relevant. There definitely isn't a time window in which Bob might get a different answer, so the breaking of the entanglement doesn't propagate slowly in that sense.

Does the measuring process of Alice happen instantaneously? Or is it like decoherence would suggest - continuous but in a very short time. During that time what happens to the entanglement with the other particle?

Thanks
Bill
 
  • #80
bhobba said:
Does the measuring process of Alice happen instantaneously? Or is it like decoherence would suggest - continuous but in a very short time. During that time what happens to the entanglement with the other particle?

Let's suppose that Alice's measurement starts at time ##t_1## and finishes at time ##t_2##, and let's suppose that Bob's starts at ##t_3## and finishes at ##t_4##. If Bob is far enough away from Alice so that there is no possibility of a light-speed or slower signal propagating from Alice at time ##t_1## to Bob at time ##t_4##, then I don't see what difference it makes how long Alice's measurement took.
 
  • #81
stevendaryl said:
Let's suppose that Alice's measurement starts at time ##t_1## and finishes at time ##t_2##, and let's suppose that Bob's starts at ##t_3## and finishes at ##t_4##. If Bob is far enough away from Alice so that there is no possibility of a light-speed or slower signal propagating from Alice at time ##t_1## to Bob at time ##t_4##, then I don't see what difference it makes how long Alice's measurement took.

Well let's be more precise. Suppose via slow transport Bob and Alice have syced clocks. And they both at exactly the same time observe the system (remember until entanglement is broken it is a single system). What happens then? That may be interesting to both analyse and do. I wonder if @DrChinese knows anything about that or has some papers to post.

My guess is its an entirely different setup - the observable will be a compound observable of observing both 'parts' of the entangled system which is different than what goes on in EPR.

Thanks
Bil
 
  • #82
bhobba said:
Well let's be more precise. Suppose via slow transport Bob and Alice have syced clocks. And they both at exactly the same time observe the system (remember until entanglement is broken it is a single system). What happens then? That may be interesting to both analyse and do. I wonder if @DrChinese knows anything about that or has some papers to post.

I don't know what tests have been done along those lines, but I'm willing to bet that it doesn't make any difference whether Bob's measurement is at the same time as Alice's, or slightly earlier, or slightly later.
 
  • #83
stevendaryl said:
I don't know what tests have been done along those lines, but I'm willing to bet that it doesn't make any difference whether Bob's measurement is at the same time as Alice's, or slightly earlier, or slightly later.
That is correct because the singlet state tells us nothing about times or time-ordering, so we can say nothing about those times.

The singlet state is also silent on states before the measurement, so nothing is ruled out. Even the pair having a fixed value. It is irrelevant because of the imminent re-projection.
 
  • #84
bhobba said:
There is no ambiguity. He says the state changes unpredictably. Nobody disagrees with that. Its the other baggage associated with collapse that is the issue.

MW proves it does not have to happen using non-unitary changes and instantaneously, nor does the formalism require it to be. It may be like that or not - we do not know. It may be like GRW - again we do not know. The formalism is silent on it.

Thanks
Bill

That is not correct. MWI and GRW are research directions on which consensus has not been reached in the community. Even supporters of MWI like Carroll and Wallace state that it has open problems. It is misleading false advertising to place them on the same level as textbook physics. This false advertising also does not benefit those research programmes, since if the issues are settled, we should now stop research into MWI and GRW.
 
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  • #85
bhobba said:
Does the measuring process of Alice happen instantaneously? Or is it like decoherence would suggest - continuous but in a very short time. During that time what happens to the entanglement with the other particle?

Thanks
Bill

That is not correct. Decoherence does not solve the measurement problem. Within the standard formalism, if one includes decoherence, the appearance of the measurement result still needs an instantaneous "measurement" on the measurement apparatus. One has to go to something like MWI for decoherence to remove collapse, but MWI is not yet textbook physics.
 
  • #86
atyy said:
That is not correct. Decoherence does not solve the measurement problem. Within the standard formalism, if one includes decoherence, the appearance of the measurement result still needs an instantaneous "measurement" on the measurement apparatus. One has to go to something like MWI for decoherence to remove collapse, but MWI is not yet textbook physics.

Is there not an observable of the composite system, system + apparatus + rest of the universe, that, if measured, would indicate whether system + apparatus + rest of universe is in a superposition or not?
 
  • #87
StevieTNZ said:
Is there not an observable of the composite system, system + apparatus + rest of the universe, that, if measured, would indicate whether system + apparatus + rest of universe is in a superposition or not?

Basically, there is no rest of the universe, because the rest of the universe excludes the final measurement apparatus. So if we include a measuring apparatus in the quantum state, we need another measuring apparatus to measure the first apparatus, otherwise no measurement outcome is obtained.

This is, as you know, the measurement problem, which remains unsolved. I think it is an important problem, but approaches to the measurement problem should not be brought up (as Peter Donis and bhobba did) in a thread which only refers to and makes sense within standard QM.
 
  • #88
atyy said:
a thread which only refers to and makes sense within standard QM.

I don't think we have agreement on this point. Your position appears to be that simply saying "wave function collapse isn't the same as black hole information loss" is enough to answer the OP's question. But the OP's question was whether information is lost in wave function collapse; the fact that the OP also brought in a mistaken analogy with black hole information loss does not mean his question was only about whether wave function collapse and black hole information loss are the same.

As I've already said, I don't think the question of whether information is lost in wave function collapse is answerable within standard QM. The question of whether wave function collapse is the same as BH information loss is answerable within standard QM (the answer is that the two are not the same), but, as above, that's not a complete answer to the OP's question.
 
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  • #89
PeterDonis said:
I don't think we have agreement on this point. Your position appears to be that simply saying "wave function collapse isn't the same as black hole information loss" is enough to answer the OP's question. But the OP's question was whether information is lost in wave function collapse; the fact that the OP also brought in a mistaken analogy with black hole information loss does not mean his question was only about whether wave function collapse and black hole information loss are the same.

As I've already said, I don't think the question of whether information is lost in wave function collapse is answerable within standard QM. The question of whether wave function collapse is the same as BH information loss is answerable within standard QM (the answer is that the two are not the same), but, as above, that's not a complete answer to the OP's question.

If you read the OP and his clarifications in subsequent posts, you can see that he is asking for an answer within standard QM. He is aware of still speculative approaches beyond standard QM.
 
  • #90
@PeterDonis, just to clarify - I am not objecting to the discussion of interpretations as one part of the answer to this thread. I am objecting in interpretations being brought up as a primary answer, and as if MWI is part of standard QM or that MWI has anything to do with the most common attempts (like AdS/CFT) to restore unitarity in the black hole information paradox.

If after those points have been discussed in standard QM, I do think it is perfectly fine to mention that more generally there is the measurement problem etc. Personally, I would not bring it up, since I prefer to have fewer discussion on interpretations in QM forum, and I don't like that every time collapse is brought up in the colloquial, innocuous sense of the word referring to standard QM, that interpretations are brought into the discussion. However, if no physics errors are made, I usually try (I confess, not always successfully :oops:) to shut up. Here I entered the discussion to clarify that MWI is not part of standard QM and that MWI has nothing to do with the most common attempts (like AdS/CFT) to restore unitarity in the black hole information paradox.

[I think you agree, but bhobba entered the discussion on a post in which I was replying to you, and reintroduced the erroneous idea that MWI is part of standard QM].
 

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